The invention relates to a method of and device for the manufacture of dimensionally stable, liquid-impermeable, flexible, elastic pressbands, particularly for use with broad-nip shoepresses of paper making machines, said method comprising pouring a flowable mixture of a prepolymer onto a substrate moved by a distance maintaining device over a cylindrical mandrel and maintained during this movement in an adjustable distance from the surface of said mandrel, and being coated during this movement by a coating device with a flowable prepolymer, wherein during this coating of the substrate threads or wires are laid down onto the substrate in the direction of rotation of the mandrel and are coated with the polymer substance.
Such method is known from the German published patent application DE 40 22 800. The pressbands and presscoats for wet presses manufactured by this known method, are disadvantageous, however, because of the fact that the longitudinal threads and cross threads embedded in the coating material are slowly released from the coating material under the pressure forces and tensional forces created within the press band during operation of such a wet press, if the tensile modules of threads, adhesion promoters and coating material had not been precisely checked up to one another previously. Such a check up, however, is rather difficult. Moreover, the tensile modules are changing because of ageing. As a consequence thereof the so-called excavating or tunneling effect arises according to which the threads are providing around them hollow spaces because of the alternating tensions affecting them. This effect, however, essentially decreases the strength of the band substantially and thus also its lifetime or durability and destroys in this connection especially the original tensile strength of the band.
Moreover, pressbands manufactured according to the known method are often destroyed during operation in case the paper web is drawn into the nip together with waste material causing a local breaking of the pressband below the shoe, whereas the elastic felt web proceeds on the non-breaked areas of the pressband. That is the reason why the pressband is generally so strongly deformed in the range of failure that it breaks in cross-direction as soon as the deformation forces cannot be taken over by the pressband.
It is therefore an object underlying the invention to provide a method of the initially specified kind in such a way that the manufactured pressbands have the required high strength and tensile stability and that they are able to maintain both over very long times of operation.
It is a further object of the invention to improve the method of manufacturing insofar as the pressbands remain rather unaffected in case of local deformations of such pressbands caused by waste material in the nip of the shoepress.
It is still a further object of the invention to provide a device which is designed in such a way as to permit a highly economic and therefore inexpensive performing of said method and in this connection also an improved pressband should be manufactured.
These and other objects are accomplished according to one aspect of the invention by providing a net, texture, knitted fabric or a similar structure as substrate tensioned over the surface of the mandrel and consisting of non-woven threads located transverse to the longitudinal direction of the pressband and forming meshs the width thereof is chosen dependent especially on the viscosity of the prepolymer mass such that the meshs are completely impregnated by the prepolymer mass poured onto the meshs and that the threads of the net, texture, knitted fabric or a similar structure are completely coated by the mass forming on the side opposite to the surface of the mandrel a closed, smooth surface of prepolymer material, the thickness of which corresponds to the distance of the substrate from the surface of the mandrel, which polymer material can be separated from said mandrel surface.
By that positioning of the net, texture, knitted fabric or a similar structure within the pressband in relation to the running direction of the pressband, as well as by the fact that the threads those nets, textures, knitted fabrics or similar structures are consisting of are connected to one another but not interwoven with one another, the flexibility or elasticity of the pressband on operation is improved essentially so that a tension profile is formed which avoids the creation of tension peaks and which is in contrast to the tension profile of the common pressbands more uniform and more homogenous and in addition to that it is on a lower level. As a consequence the crossthreads and longitudinal threads of the substrate which are with respect to the direction of running of the pressband no more positioned in parallel or rectangularly but in general diagonally are not released even after a long running time from the coating mass, for instance polyurethane, surrounding them but remain in a fixed position within the coating mass so that formation of breakes within the pressbands are completely avoided even if there will be troubles because of waste material.
There are many advantageous embodiments of the subject invention which are dealt with in the subclaims. According to one advantageous embodiment the substrate is configured as a tube and is fixed in a tensioning device at both front sides of the mandrel and is axially tensioned by a stretching device, the construction being such that the substrate is held during the coating operation in the required distance from the surface of the mandrel by a distance annulus.
In accordance with an advantageous further improvement of the invention multifile threads are used for helically winding the threads onto the substrate during coating of the same by casting the prepolymer onto the surface of the substrate resulting in obtaining the required strength of the pressband in circumferential direction thereof.
According to a further advantageous feature of the subject invention the threads of which the substrate is provided, i.e. the net, texture, knitted fabric or similar structure, can be multifile threads of plastic material in order to obtain the required tensional stability of the pressband.
After the casting of the prepolymer material and its couring on the surface of the mandrel the obtained pressband can be advantageously mechanically treated to obtain the required thickness and/or quality of the surface before the pressband will be withdrawn from the surface of the mandrel. In this connection it has been found further advantageous to provide the surface of the pressband also with holes and/or depressions for the removal of liquid.